The present invention relates to a method for controlling the defrost of an evaporator in a refrigeration appliance having a cooling circuit with a variable cooling capacity compressor. The present invention relates as well to a refrigeration appliance having a cooling circuit with a variable cooling capacity compressor and a control unit for driving said compressor.
With the term “variable cooling capacity compressor” it is meant every kind of compressor having a control unit adapted to adjust the cooling capacity thereof. Even if the term applies generally to compressors having a variable speed motor according to the different frequencies of the inverter system driving the compressor, the term applies to on/off compressors as well, in which the cooling capacity control is carried out by adjusting the percentage of compressor insertion.
EP1619456 discloses an algorithm allowing estimation of one or more thermodynamic parameters from one or more thermodynamic or electrical parameters of cooling system circuits in which a cell temperature is used for feeding an estimating algorithm which can control the compressor.
The control process disclosed by EP1619456 can be sketched as shown in FIG. 1, in which a couple of inputs to the control system are required, i.e. an input from one of the actuators and an input form a temperature sensor. This approach is quite complex and therefore the overall cost of the control system is high.
Currently no estimation is made on cooling circuit, and only heat exchanger measurements are available. Therefore current adaptive defrost algorithms cannot take in account the effect of variable cooling capacity supplied by the compressor into the frost creation process. The known algorithm make the assumption that frost generation is constant in respect of compressor run time. In the variable cooling capacity compressor this is not the case. A system control, taking advantage of variable cooling capacity compressor, could select cooling capacities lower than maximum values allowed. This would cause the frost creation process to be slower than the frost generated by a single speed compressor driven in the same way. An algorithm that doesn't take into account this factor tends to estimate the needing of a defrost procedure more frequently than necessary. This results in lowered performances.
One object of the present invention is to provide a method for controlling defrost of an evaporator which is highly reliable and needs few component or sensors. Another object is to provide a method which is particularly useful for controlling the defrost timing in refrigerators using variable cooling capacity compressors. Another object is to provide an algorithm that avoids an overestimation of frost buildup, particularly when a variable cooling capacity compressor is used.
The above objects are reached thanks to the features listed in the appended claims.